1. Field of the Invention
The present invention relates to a powdery toner used for visualizing a latent image formed on a photoconductor in electrophotography, electrostatic recording, etc., and a method for producing such a toner. More particularly, it relates to a toner suitable for a nonmagnetic one-component developing method which can be conveniently used for compact printers, plain paper facsimiles, etc., and a method for producing such a toner.
2. Discussion of the Related Art
In conventional methods in electrophotography, electrostatic recordings, etc., as the most convenient method for visualizing the latent image formed on the photoconductor using a powdery toner, there have been proposed two-component magnetic brush developing methods using a developer consisting of two components, namely, a toner and a carrier, the carrier being used for the purposes of supplying electric charges to the toner and of conveying the charged toner onto the latent image portion by a magnetic force.
However, in the two-component magnetic brush developing method, since a magnetic force is utilized in the conveying of the developer, a magnet has to be placed in the developer roller, and the carrier is made of a metal or an oxide thereof such as iron powder, nickel powder, and ferrite. Therefore, the developer device and the developer become undesirably heavy, thereby making it difficult to miniaturize and thus reduce the weight of the overall recording device.
On the other hand, as disclosed in U.S. Pat. Nos. 3,909,258 and 4,121,931, there have been conventionally well used magnetic one-component developing methods comprising the step of conveying a toner to the latent image portion without using a carrier, the methods being carried out by utilizing a magnetic force owned by the toner containing a magnetic substance therein. However, a magnet has to be also used in the inner portion of the developer roll in this developing method, making it difficult to reduce the weight of the developer device.
In order to solve the problems in these developing methods, much investigations have been recently conducted on nonmagnetic one-component developing methods wherein a toner alone is used without containing any magnetic powder, as disclosed, for instance, in U.S. Pat. Nos. 2,895,847 and 3,152,012, and Japanese Patent Examined Publication Nos. 41-9475, 45-2877 and 54-3624.
However, in the conventional nonmagnetic one-component developing methods, since toners are provided with electric charges only at an instant when the toner passes near the charging blade, the charging control of the toner in these methods is extremely difficult. In order to solve this problem, there have been proposed a method in which a silica fine powder surface-treated with a titanate coupling agent, the silica fine powder having a particle diameter of 1 to 2000 nm, is added to the surface of the toner containing a styrene-butadiene copolymer as a binder resin (see, for instance, Japanese Patent Laid-Open No. 59-231549); and a method in which a particular charge control agent is used (see, for instance, Japanese Patent Laid-Open No. 63-226666).
On the other hand, it is also important to improve the contact efficiency of the toner with the charging blade. In order to achieve good contact efficiency, various external additives have been investigated, as disclosed, for instance, in Japanese Patent Laid-Open Nos. 64-77075, 3-294864, etc. Further, for the purpose of solving the problem of poor charging of small toners passing besides large toners, a particle diameter distribution has been also investigated as disclosed, for instance, in Japanese Patent Laid-Open No. 63-279261.
However, in the above methods, although toners may provide good fixed images at start, during repeated copying and supplying of the toner, such problems arises that the image density lowers, that the background increases and that the resolution of formed image lowers.
In the meantime, Japanese patent Laid-Open No. 3-294864 discloses that inorganic fine particles having a particle diameter of 0.1 to 1.0 .mu.m are added to the toners. However, the inorganic fine particles in this reference are added for the purpose of polishing the surface of photoconductor to remove toner filmings formed on the photoconductor.